The Internal Model Control (IMC) structure has been widely recognized as very useful in clarifying the issues related to the mismatch between the model used for controller design and the actual process. The structure also gives rise to a two step controller synthesis procedure, of which the second step deals with the design of a low pass filter such that robustness with respect to model-plant mismatch is guaranteed. The Structured Singular Value (SSV) was introduced recently and it allowed the non-conservative quantification of the concept of robust performance. This paper deals with the design of the IMC filter by uaing the SSV and it demonstrates how this approach can be used with either an H{SUB 2^-} or an H{SUB INFINITY} optimal controller.